In the Session Initiation Protocol (SIP), a dialog for a communication is setup for different kinds of communications sessions, such as voice, video, text, and the like. The initial signaling to establish the dialog for the communication session is typically setup using a SIP proxy server and/or a Back-to-Back User Agent (B2BUA). Once the communication session is setup using the SIP Proxy server and/or the B2BUA, a media session may be created directly between user agents in SIP devices. For example, a SIP proxy server can setup an initial voice call between two SIP devices. Once the initial voice call is established by the proxy server, the communication session voice call can be setup (e.g., using Real-Time Protocol (RTP)) directly between the two communication devices.
This call setup model creates some problems if the SIP Proxy server in the above example fails during the communication session. For example, if a user of a SIP device involved in the communication session wants to transfer a voice call, the user of the SIP device will be unable to transfer the voice call even if the SIP proxy server has been reinitialized or failed over to a new SIP proxy server. This is because the reinitialized/new SIP proxy server is unaware of the previous voice call.